Oxidizing halogen compounds, specifically, chlorine, bromine and iodine are known to degrade in the sunlight-ultraviolet (UV) spectral region (hereinafter referred to as "sunlight-UV").
The sunlight-UV degradation of halogens is a serious problem in swimming pools, spas, and hot tubs and can also be a serious problem in certain industrial water treatment applications, such as decorative fountains, settling and cooling ponds, clarifiers, etc. Without the addition of a proper halogen stabilizer, over 50% of the halogen will decompose in less than half an hour under normal sunlight illumination.
Based on the principles of photochemistry, for an oxidizing halogen compound to be degraded under UV illumination, two process steps must occur. The first step is UV absorption of the molecule by which the molecule is excited to an higher energy state. In the second step, the excited molecule undergoes a photodegradation reaction. By understanding the mechanism of this reaction, two approaches can be taken to prevent a halogen from undergoing photodegradation. The first approach is to look for an oxidizing halogen compound that has minimal light absorption in the UV range (sunlight-UV is from 290 to 400 nm). The second approach is to look for an oxidizing halogen compound which even at the excited state does not undergo a photodegradation reaction, but rather undergoes vibrational relaxation to return to ground state. For example, unstabilized bromine has a maximum absorption at 330 nm. However, a stabilized bromine solution may have different light absorption characteristics depending on the properties of the stabilizer and degree of stabilization. The lower the absorption in UV range, the more stable the bromine will be.
Bromine is known to be an excellent biocide in recreational water treatment. However, the use of bromine has been severely limited in outdoor swimming pools due to its rapid degradation in sunlight-UV. There is no known practical means to protect bromine from such degradation. Therefore, bromine is limited to only about 10% of the specialty biocide market share and is used principally for indoor pools and spas. Bromine is preferred for pool use because antimicrobial performance is superior to chlorine and bromine is less irritating to soft tissues and membranes, such as in and around the eyes. U.S. Pat. No. 3,493,654 (Goodenough et al.) describes the use of succinimide as a stabilizer to increase the half-life of bromine in UV illumination. However, although some UV stability is achieved, none of the stabilizers disclosed in the Goodenough et al. patent are used commercially because they are either not practical or effective enough.
Currently, degradation of chlorine, but not bromine, can be substantially prevented in sunlight-UV by using an organic stabilizer such as triazine-s-trione (cyanuric acid). Commercially, chloroisocyanurate is the principal specialty biocide used in pools for microbiological control (approximately 90% of the specialty biocide market share) because the cyanurate stabilizer protects chlorine from degradation by sunlight-UV. However, cost is a factor because, to be effective, large amounts of the cyanurate stabilizer are required. Typically, at least 35 parts per million (ppm) of cyanurate must be used for every 1 ppm of chlorine.
According to the Kirk Othmer Encyclopedia of Chemical Technology, Third Edition, Wiley Interscience, Volume 24, pages 427-441, the recommended initial cyanuric acid concentration is 50 ppm. However, if the concentration goes above 100 ppm, partial drainage of the pool water and refilling with fresh water will be required because the excessively high cyanuric acid concentration may slow down the rate of disinfection. Therefore, the operational window for cyanuric acid is quite narrow (a concentration factor of 2).
Accordingly, it would be desirable to develop a biocide composition for use in treating recreational and industrial water systems which is sunlight-UV stable, economically-appealing and has a wide operational window.